1. Field of the Invention
The present invention relates to a load test for a press and, more particularly, to a technique for detecting the actual state of a mechanical press.
2. Prior Arts
In hydraulic presses, there is almost no problem in measuring load since a load applied during operation of the press can be directly and visually detected by an oil pressure gauge, because the speed at which load is applied is not high. On the other hand, in mechanical presses, visual detection of the dynamic load is not easy as compared with hydraulic presses, because the load is applied with a strong impact. Besides, there are further essential problems incident to presses of this type.
With regard to a conventional measuring device mounted on a mechanical press, it has been heretofore known to employ means in which a load cell (strain gauge or dial gauge) is secured to a frame or post of the press, and by which, in a load test, a strain produced in the mechanical structure, such as the frame of the press, is measured, or variations in pressure produced in the incorporated hydraulic system are detected and electrically converted, thereby finally detecting the applied load. With such conventional means, however, a serious problem arises and must be solved with a view toward an accurate measurement of the load. This is so because dynamic load is usually applied with a strong impact. Moreover, the shape of a pressed product to be transferred to a die is not always lengthwise and breadthwise symmetrical, but is at times unevenly one-sided in most presses and, therefore, it may be said that, in most cases, a load is not applied uniformly but applied rather eccentrically in the form of a so-called eccentric load.
Further, in forging presses, multistage production dies are popularly employed, in which molds for two or more stages are arranged in one die and a material to be forged is timely transferred with the advance of the forging process. In such a multistage production forging die, the mentioned eccentric load takes place in all of the forging stages except the center part. Without accurately detecting such eccentric loading to adjust the bottom dead point appropriately, there is a possibility that an unexpected overload will be partially applied, eventually resulting in strain and/or cracking of the equipment and/or material. In spite of such a possible danger of breakdown, accurate measurement of the eccentric load is very difficult, which is another problem to be solved. Several attempts have been proposed to solve the mentioned problems.
For example, in the Japanese Laid-Open Patent Publication (unexamined) No. 54-42081, as illustrated in FIG. 3, a load measuring unit 102, on which a load cell 101 is placed and which is provided with a strain enhancement function, is prepared; then two posts located at positions of point symmetry with respect to the center point of the cross-section of a mechanical press are selected; two positions of point symmetry with respect to the center point of the cross-section of each post are further selected; a load unit like that mentioned above, is then mounted at each of the selected positions; and the output from each unit is transmitted to a dynamic strain type of load meter to measure a forging load. Publication No. 54-42081 discloses that, as a result of employing the mentioned arrangement, the conventional problem of inaccurate load measurement, which may lead to erroneous adjustment of the bottom dead point and sometimes result in press mechanism breakdown, can be successfully prevented, and load measurement has been improved to the extent of enabling detection of an accurate value of an eccentric load.
In the case of a mechanical press, however, a load is applied with strong impact in a moment, and therefore it is essential as a fatally important requirement to measure such a momentary load accurately and adjust the bottom dead point appropriately.
It is to be noted that the load measuring device is secured to any part of the press body, and accordingly, not only the press body but also the load measuring device itself will receive a strong shock and a high temperature, both incidental and peculiar to mechanical presses. This means that the load measuring device, being a delicate precision instrument, is exposed to the hardest condition as compared with other precision instruments and, therefore, there still remains a further problem that the reliability of a value measured by the load measuring device is lost in a rather short time with the operation of the press. The prior art mentioned with reference to FIG. 3 is not free from this problem, either.
To meet this problem, and overcome such a possibility of losing reliability, maintenance work is an essential requirement because working a press safely cannot go on without securing at all times the reliability of the mounted measuring device itself. Thus, in a working spot of the forging, where quality control is advanced, accurate load detection must be carried out repeatedly by inserting a separate hydraulic press into a portion of a mechanical press where load is applied.
FIG. 4 illustrates a press body associated with a conventional method for detecting a measured load. In this press body, a test hydraulic cylinder 103 is inserted between a slide 2 and a bed 3 of a mechanical press body 1, and the hydraulic cylinder 103 is provided with a sensor 104, a hydraulic device 105 and a measuring device 106, thereby forming a measuring circuit. With such an arrangement, the mechanical press 1 is put into operation, then a load thereof is transmitted to the hydraulic cylinder, and a value obtained which is converted and outputted to the measuring device.
In a test of this kind, not only expert techniques but also special equipment are required. Normally, it is nearly impossible to have such equipment in an average press working plant. In actuality, therefore, service engineers are sent by the manufacturer of the mechanical press, when requested by the user, to provide the mentioned kind of test service. Accordingly, there arise unavoidable problems such as the cost of the test service, loss caused by the suspension of routine operation to carry out such a test, shortage of skilled measuring service engineers, etc., eventually resulting in difficulty of carrying out sufficient maintenance of the press. Further, in the case of a forging press of large load, a test hydraulic cylinder of large capacity is required, and other auxiliary instruments must also be large corresponding to such a large capacity. On the part of the manufacturer, it may be a heavy burden to assort various kinds of test hydraulic cylinders and take them to the user's facility. Furthermore, in the case of forging presses, it is an essential requirement to measure and obtain the mentioned eccentric load, particularly in a multistage die. However, in carrying out such a measurement, there is actually no room for installing the axis of the test hydraulic cylinder of large capacity just on the eccentric position of the measurement, thus there is a spacial restriction in the measurement of the eccentric load, which is a further problem to be solved.